It is generally known to reinforce carbon fiber skin panels with carbon fiber stringers in order to withstand the high loads that an aircraft has to withstand during flight, take offs and landings. Skin panels of an aircraft such as fuselage skin panels, are usually reinforced with several types of stringers: T, U, J and omega-shaped stringers.
Omega stringers have generally a trapezoidal configuration in a cross-sectional view, with two lateral extensions called stringer foot through which the stringer is joined to the skin panel. The omega stringers can be pre-cured and adhesively bonded with the panel (co-bonded), or alternatively the stringers and the skin panel are co-cured together.
In one alternative for manufacturing these stiffened skin panels, cured stringers or un-cured pre-formed stringers are first placed in respective grooves formed on a male tool or a mandrel, as for example a barrel type mandrel for the manufacture of a fuselage section of an aircraft, and the a skin panel is formed on the mandrel by laying up a plurality of composite plies to form a laminate, by known automated process. Alternatively, a female tool can be used instead of the male tool, and in this case the skin is first laminated on the female tool, and then cured or un-cured stringers are conveniently placed on the laminate.
Finally, the skin panel and the stringers are co-bonded together if cured stringers have been used, or alternatively the skin and the stringers are co-cured together in the case that un-curd stinger have been used.
Due to the shape of the omega stringers, closed channels are formed between the omega stringers and the skin panel to which the stringers are joined, thereby internal surfaces are formed alone those channels which also need to be consolidate gad from the inside of the stringer. Usually for consolidating these internals parts of the structure during curing, tubular vacuum bags are paced inside the closed channels extending lengthwise. These tubular vacuum bags are typically made of a flexible thin film, and are placed inside the stringers in a contracted or deformed state, that is the outer surface of these bags is completely wrinkled.
Autoclave pressure is then applied inside these tubular bags, which causes the bag to inflate to exert pressure on those internal surfaces of the stringer. The process of using these tubular vacuum bags is affected by the following drawbacks:                bad superficial quality of the inner surface of omega stringer. Even if the tubular vacuum bag expands due to the autoclave pressure being applied to its interior, the outer surface of the tubular vacuum bag cannot adapt completely to the internal surface of the stringer, especially at its corners. This means that some wrinkles remain on the surface of the vacuum bag during the curing process, which causes the formation of resin wrinkles on those internal surfaces.        manual operations are needed to correct superficial defects due to resin wrinkles. To correct those defects, sections of the omega stringers where a resin wrinkle is detected, need to be cut out and replaced by a new stringer section.        high probability of breaking the tubular bags while they are being inflated and subsequent extraction of the bags.        